The present invention relates to a method of refining tall oil.
In a method such as this, the crude tall oil is fed into a column distiller in which it is separated into desired fractions, at least part of which are recovered as product flows.
Fibers, lignin and other wood components, such as fatty acid fractions and resin acid fractions as well as neutral extractives, are released associated with the process of pulping. After washing, the cellulose is mainly used for papermaking and the lignin fraction is generally burned, whereas the cooking chemicals are recovered and regenerated. The residual liquor of the sulfate cooking (black lye) is vaporized before the burning, and the extractives and the tall oil soap are recovered from the surface of the partly concentrated residual liquor, following the fourth vaporization stage. The recovered tall-oil soap is acidified and, as a result, the fatty acids and the resin acids are released and with them the neutral extractives are carried forward in the process.
The acid, the water and the lignin as well as the accompanying fiber residues are returned to the regeneration processes of the pulping and the crude tall oil is processed by distillation at a low pressure.
Different solutions have been developed for regulating the distilling processes in the processing industry. In the literature, a general control method for column distillers is known in which the bottom efficiency or the bottom number of the entire column, both of which depend on the gas flow and the reflux ratio, is taken into account in the calculations. By adjusting these, the distillation can be controlled, for instance when the feed is changed.
In improved methods, the distillation is controlled by simulation and by analyzing the product and feeding flows of the distillation. Thus, U.S. Pat. No. 6,088,630 describes a means to control the distillation, a means that is based on the idea that the process is simultaneously measured and simulated, and the parameters that are obtained as a result of these functions are changed so that the parameters used in the simulation are average temperatures of the columns. It is proposed that the method can be employed in cases where the measuring of the average temperatures of the columns replaces expensive and time-consuming gas chromatography analyses and/or mass spectrometry analyses. According to the example in the patent publication, the average temperatures from nine different bottoms are measured in the distillation of toluene and xylene, and based on these measurement results the concentration profiles are calculated and the reboiling and the reflux ratio are adjusted so that the desired calculated concentrations are achieved.
It is obvious that a method like this is only suitable in cases with a substantial difference between the boiling points of the products to be distilled.
Another patent, U.S. Pat. No. 5,343,407, describes a non-linear model for controlling the distillation. In this model, the distillate, the bottoms product and the feed are analyzed and on the basis of the analysis results the bottom efficiencies of the distillation at the moment of the analysis are recalculated, assuming that the vaporabilities of the components are known and they are constant. From these parameters the bottom boiler capacity and the distillation reflux ratio are recalculated and reoptimized.
According to the examples of the patent publication, the purpose of the solution described above is mainly to prepare monomers that are very pure and can be analyzed quickly, for instance by means of gas chromatography (due to the low molecular weight of the compounds).
The situation whereby tall oil is distilled is quite different from the distilling of small molecular hydrocarbons. The problems are associated with, among others, to the analyzing of product flows and to real time collecting of desired information about those flows. The bulk of tall oil components are resin and fatty acids, whereas a smaller percentage consists of neutral extractives. Analyzing of the components, which are in acid form, by means of gas chromatography generally requires silylation or methylation of the compounds, which makes it difficult to analyze those components directly during the process. In such cases, the columns of the gas chromatography equipment generally endure only a few weeks of continuous use.
A direct “on-line” analysis is significantly more successful when a mass spectrometer is used. However, mass spectrometers are expensive and demand continuous expensive service.
As a result, it is difficult to obtain the measurements that are needed in known controlling methods for distillation of tall oil and corresponding natural products.
The purpose of the present invention is to eliminate the disadvantages associated with known technology, and to generate an entirely novel solution for the controlling of distillation of crude tall oil and thus for producing tall oil products.
In this method, the crude tall oil is fed conventionally into a fractionating column distiller, in which it is separated into the desired fractions, at least part of which are recovered as a product flow. The conditions of the column distiller are in this way regulated on the basis of the composition of the feed and the product flow.
The invention is based on the idea that instead of the flows being analyzed in the laboratory, in order for the compositions to be continuously determined, the density of the flows in process is measured in real time using a high measurement and reading accuracy.